iommu/vt-d: Simplify intel_irq_remapping_select()

Now that the old get_irq_domain() method has gone, consolidate on just the
map_XXX_to_iommu() functions.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-31-dwmw2@infradead.org